C# windows phone 8.1中后台任务上显示对话框的问题

我有这个代码在后台执行httpwebrequest和response方法，我只想在下载zip崩溃时显示对话框以获取信息，我的代码输入到这个捕获

    private void DoSincroFit()
    {
        HttpWebRequest request = HttpWebRequest.CreateHttp(url);
        request.BeginGetResponse(new AsyncCallback(playResponseAsync), request);
    }

    public async void playResponseAsync(IAsyncResult asyncResult)
    {
        //Declaration of variables
        HttpWebRequest webRequest = (HttpWebRequest)asyncResult.AsyncState;

        try
        {
            string fileName = "sincrofit.rar";

            using (HttpWebResponse webResponse = (HttpWebResponse)webRequest.EndGetResponse(asyncResult))
            {
                byte[] buffer = new byte[1024];


                var newZipFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);
                var newZipFile = await KnownFolders.DocumentsLibrary.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

                using (var writeStream = await newZipFile.OpenAsync(FileAccessMode.ReadWrite))
                {
                    using (var outputStream = writeStream.GetOutputStreamAt(0))
                    {
                        using (var dataWriter = new DataWriter(outputStream))
                        {
                            using (Stream input = webResponse.GetResponseStream())
                            {
                                var totalSize = 0;
                                for (int size = input.Read(buffer, 0, buffer.Length); size > 0; size = input.Read(buffer, 0, buffer.Length))
                                {
                                    dataWriter.WriteBytes(buffer);
                                    totalSize += size;    //get the progress of download
                                }
                                await dataWriter.StoreAsync();
                                await outputStream.FlushAsync();
                                dataWriter.DetachStream();
                            }
                        }
                    }
                }

            }
        }
        catch
        {
           SMethods.Message_Dialog("Download has stopped!","Error");
        }
    }

但是当我的代码执行这个方法时，从这个类：

class StandarMethods
{
public async void Message_Dialog(string text, string title)
    {
        //Declaration of variables
        MessageDialog MDialog = new MessageDialog(text, title);

        await MDialog.ShowAsync();
    }
}

最后，我的应用程序在尝试执行时崩溃：

await MDialog.ShowAsync();

---

这是在后台等待的任务。。。有人能帮我吗？是否有可能使用事件处理程序？为什么？怎么用？提前谢谢

Merli您的问题是您试图从后台访问UI线程，以便向用户显示对话框，因此使用Dispatcher进行此基本示例是：-

// This is for silverlight part
    Deployment.Current.Dispatcher.BeginInvoke(delegate
    {
      var mbr = MessageBox.Show("Are you sure you want to leave this page?", "Warning",      
      MessageBoxButton.OKCancel);

      if(mbr == MessageBoxResult.OK)
      {   OK pressed  } 
      else
      {   Cancel pressed  }

    });

对于winrt部分-

CoreDispatcher dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
    dispatcher.RunAsync(CoreDispatcherPriority.Normal, async()=>{
          // UI code goes here
          //Declaration of variables
        MessageDialog MDialog = new MessageDialog(text, title);
        await MDialog.ShowAsync();
    });

我的最终代码：

    private void DoSincroFit()
    {
        HttpWebRequest request = HttpWebRequest.CreateHttp(url);

        //Add headers to request
        request.Headers["Type"] = "sincrofit";
        request.Headers["Device"] = "1";
        request.Headers["Version"] = "0.000";
        request.Headers["Os"] = "WindowsPhone";

        request.BeginGetResponse(new AsyncCallback(playResponseAsync), request);
    }

    public async void playResponseAsync(IAsyncResult asyncResult)
    {
        //Declaration of variables
        HttpWebRequest webRequest = (HttpWebRequest)asyncResult.AsyncState;

        try
        {
            string fileName = "sincrofit.rar";

            using (HttpWebResponse webResponse = (HttpWebResponse)webRequest.EndGetResponse(asyncResult))
            {
                byte[] buffer = new byte[1024];
                var newZipFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

                using (var writeStream = await newZipFile.OpenAsync(FileAccessMode.ReadWrite))
                {
                    using (var outputStream = writeStream.GetOutputStreamAt(0))
                    {
                        using (var dataWriter = new DataWriter(outputStream))
                        {
                            using (Stream input = webResponse.GetResponseStream())
                            {
                                var totalSize = 0;
                                for (int size = input.Read(buffer, 0, buffer.Length); size > 0; size = input.Read(buffer, 0, buffer.Length))
                                {
                                    dataWriter.WriteBytes(buffer);
                                    totalSize += size;    //get the progress of download
                                }
                                await dataWriter.StoreAsync();
                                await outputStream.FlushAsync();
                                dataWriter.DetachStream();
                            }
                        }
                    }
                }

            }
        }
        catch
        {
            dispatcher = CoreWindow.GetForCurrentThread().Dispatcher; //CRASH IN THIS LINE!
            dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
            {
                //Declaration of variables
                SMethods.Message_Dialog("Download has stopped!", "Error");
            });
        }
    }

这是我的照片：

---

谢谢大家

已解决，我的最终代码如下：

    private CoreDispatcher dispatcher;

    private void DoSincroFit()
    {
        HttpWebRequest request = HttpWebRequest.CreateHttp(url);

        //Add headers to request
        request.Headers["Type"] = "sincrofit";
        request.Headers["Device"] = "1";
        request.Headers["Version"] = "0.000";
        request.Headers["Os"] = "WindowsPhone";

        dispatcher = CoreWindow.GetForCurrentThread().Dispatcher;
        request.BeginGetResponse(new AsyncCallback(playResponseAsync), request);
    }

    public async void playResponseAsync(IAsyncResult asyncResult)
    {
        //Declaration of variables
        HttpWebRequest webRequest = (HttpWebRequest)asyncResult.AsyncState;

        try
        {
            //For download file  with stream
            //http://social.msdn.microsoft.com/Forums/windowsapps/en-US/de96a61c-e089-4595-8349-612be5d23ee6/download-file-with-httpwebrequest?forum=winappswithcsharp
            string fileName = "sincrofit.rar";

            using (HttpWebResponse webResponse = (HttpWebResponse)webRequest.EndGetResponse(asyncResult))
            {
                byte[] buffer = new byte[1024];

                //For acces Local folder of phone device
                //http://social.msdn.microsoft.com/Forums/windowsapps/en-US/ec99721c-6565-4ce9-b6cc-218f2265f9c7/what-is-the-uri-of-an-isolatedstorage-file?forum=wpdevelop
                var newZipFile = await ApplicationData.Current.LocalFolder.CreateFileAsync(fileName, CreationCollisionOption.ReplaceExisting);

                using (var writeStream = await newZipFile.OpenAsync(FileAccessMode.ReadWrite))
                {
                    using (var outputStream = writeStream.GetOutputStreamAt(0))
                    {
                        using (var dataWriter = new DataWriter(outputStream))
                        {
                            using (Stream input = webResponse.GetResponseStream())
                            {
                                var totalSize = 0;

                                for (int size = input.Read(buffer, 0, buffer.Length); size > 0; size = input.Read(buffer, 0, buffer.Length))
                                {
                                    dataWriter.WriteBytes(buffer);
                                    totalSize += size;    //get the progress of download

                                    dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
                                    {
                                        //Declaration of variables
                                        pBar.Value = sizeFit / totalSize * 100;
                                    });
                                }
                                await dataWriter.StoreAsync();
                                await outputStream.FlushAsync();
                                dataWriter.DetachStream();
                            }
                        }
                    }
                }

            }
        }
        catch
        {
            dispatcher.RunAsync(CoreDispatcherPriority.Normal, async () =>
            {
                //Declaration of variables
                SMethods.Message_Dialog("Download has stopped!", "Error");
            });
        }
    }

---

感谢您的时间@loop

太好了！但是这个代码给了我一个小错误。。。错误：“Object reference no set to and intance of a Object”这是一个密封类，没有构造函数方法。知道吗？谢谢大家！此代码不起作用，因为visual studio会告诉我：“实际上下文中不存在名称“部署”，当我使用visual studio帮助时，会导出此库：Windows.Phone.Management.Deployment.Current.Dispatcher.BeginInvoke。。。但视觉告诉我电流不存在，是一个错误。最后，MessageBox控件在Windows phone 8中不存在。泰克斯！您是为WP8.1 silverlight还是WP8.1 winrt模板开发的？第一个代码是：CoreDispatcher=CoreWindow.GetForCurrentThread（）.dispatcher；dispatcher.RunAsync（corespatcherpriority.Normal，async（）=>{//变量声明SMethods.Message_对话框（“下载已停止！”，“错误”）；}）；在调度程序上赋值时，第一行失败。谢谢allI，我不太清楚，抱歉，我是wp silverlight的新手开发者，它是nuguet软件包还是库？如果是，那么我使用winrt模板，我怎么知道这一点？谢谢大家！